In diffraction gratings such as Echelle gratings having metallized facets with both non-reflective facets and reflective facets being metallized, the diffraction efficiency can be substantially different for light polarized parallel to the edges of the facet versus perpendicular to the edges of the facet. In planar waveguide based diffraction gratings, the gratings are created by etching sidewalls perpendicular to the plane of a waveguiding slab, and the facets are often metallized to create high reflectivity within the slab. In such structures, the presence of metallized sidewalls nearly perpendicular to each reflective facet creates polarization dependence in the diffraction efficiency from the facets. Effectively, the diffraction efficiency of the grating will have a different relationship versus diffracted wavelength for polarizations parallel to the plane of the slab (TE mode) relative to polarizations perpendicular to the plane of the slab (TM mode). This difference in diffraction efficiency with reference to polarization state leads to Polarization Dependent Loss (PDL).
One suggested technique to reduce PDL, which is taught in U.S. Pat. No. 6,400,509, relies on the selective removal of metal from non-reflective facets or preferential metallization of the entire reflective facet.
Assuming that selective removal of metal from non-reflective facets is employed, such techniques are likely to be cumbersome (because selectivity between the reflecting and non-reflective facets is hard to ensure), especially if the grating is concave as in the case of a Rowland circle design. In reality, such techniques may not be able to remove all the metal from the non-reflective facets leading to non-uniform metal coverage and hence uncontrolled wavelength dependent PDL (Polarization Dependent Loss) or PDλ (Polarization Dependent wavelength shift). Finally, such techniques cannot leave a predetermined degree of metal coverage on reflective facets.